1. Field of the Invention
The present invention relates to a semiconductor light emitting device, and more particularly to a semiconductor light emitting device with which generated light can be effectively extracted to the outside.
2. Description of the Related Art
Since light emitting diodes (hereinafter referred to as LEDs) have high reliability, they are used as light sources in various types of display apparatus as a substitute for a tungsten lamp. Furthermore, the LEDs have attracted much attention as display devices which are used indoors and outdoors. As the intensity of light emitted from the LED becomes higher, it is expected that the LED will be an alternative medium to a neon lamp in the future. In a few years, the outdoor display market of LEDs will remarkably progress. The high-intensity LED was first realized as an LED for emitting red light having a double heterostructure (DH) of GaAlAs system a few years ago.
Recently, a DH-type LED for emitting light of a band from orange to green of InGaAlP system is made so as to be of a high-intensity type. In order to increase the luminous efficiency of such LEDs, it is important to increase the generating efficiency of light within the LED and also it is important to increase the extracting efficiency of the generated light to the outside.
FIGS. 10A and 10B show an exemplary LED of the prior art. The LED is an InGaAlP-system LED 100 capable of emitting light of a green band. FIG. 10A is a transverse cross-sectional view of the LED 100 in which current flow is indicated by broken lines. FIG. 10B is a transverse cross-sectional view of the LED 100 in which light generation is indicated by solid lines (a-d).
In the LED 100, an n-GaAs buffer layer 109 (thickness: 0.1 micrometers (.mu.m)), an n-In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P carrier confining layer (cladding layer) 102 (thickness: 1.5 .mu.m), a non-doped In.sub.0.5)Ga.sub.0.55 Al.sub.0.45).sub.0.5 P active layer 103 (thickness: 0.7 .mu.m), a p-In.sub.0.5)Ga.sub.0.3 Al.sub.0.7).sub.0.5 P window layer (cladding layer) 104 (thickness: 1.5 .mu.m), a p-Ga.sub.0.3 Al.sub.0.7 As current diffusing layer 105 (thickness: 5 .mu.m), and a p-GaAs ohmic contact layer 106 are grown on an n-GaAs substrate 101, in this order, by metal organic chemical vapor deposition (MOCVD).
An upper electrode 107 is formed on the p-GaAs ohmic contact layer 106. A lower electrode 108 is formed on the lower surface of the n-GaAs substrate 101. The upper electrode 107 and the underlying ohmic contact layer 106 are formed by etching a peripheral portion thereof while leaving a portion having a size for lead bonding (width: about 70 .mu.m) unetched. A current injected from the upper electrode 107 is diffused in the current diffusing layer 105 as indicated by the broken lines in FIG. 10A. The diffused current is injected over the entire non-doped In.sub.0.5)Ga.sub.0.55 Al.sub.0.45).sub.0.5 P active layer 103, so as to generate light.
In the LED 100, the current is injected over the entire non-doped In.sub.0.5 (Ga.sub.0.55 Al.sub.0.45).sub.0.5 P active layer 103, but a larger part of the injected current concentrates in a portion 103a directly under the upper electrode 107. As a result, an amount of light generated from the portion 103a is larger as compared with the remaining portion. Regarding light beams (a-c) which travel upwardly from the portion 103a directly under the upper electrode 107, the light beams are reflected from the upper electrode 107, as is shown in FIG. 10B. Accordingly, the light beams are not extracted to the outside. Regarding light beams which travel upwardly but obliquely, if a light beam (d) is incident on an upper face 105a at a critical angle or a larger angle, the light beam is not extracted to the outside, either. Therefore, the larger part of the current injected to the active layer 103 is disadvantageously consumed for generating light which cannot be extracted.